I have to organise shipping a parcel from the seller to myself?

Yes, you can use BAC for that.

Strange seller though, I specifically mention in my listings that I won't handle courier arranged by a buyer, because they are a hassle and I can't load the shipping price with packaging, etc.

We do the same thing - "Buyer must pick up or arrange own courier" . There are a lot of reasons sellers do this, but to site one as an example - how many sellers have bad feedbacks because of slow or useless couriers? 1000's! By making it the buyers responsibility we cannot be blamed for organizing a rubbish service.

A good seller though who is expecting buyers to organize their own couriers should provide a preferred option. We recommend Pack and Send. Just do a Google search for Pack and Send in your local area and you will find a fairly easy to navigate page.

Courier post, Post Haste are also good options.

But for the love of all things holy and not, AVOID ARAMEX at all costs!!!!!

Go into courier options and go into ship something else can do it all on trademe

im buying something from a seller, but how can i send a message if they can courier/deliver before i do any payments

Ask in "Questions" . If you've already won bid you should have their email contact via confirmation of win notification. If post says "shipping to be arranged" I usually ask about approx cost of shipping in questions b4 bidding as sometimes it can be kinda expensive.